This invention relates to a document holder which can be opened and closed with respect to the document table of a device such as a copier and serves to hold an original document sheet against the document table while it is optically scanned to have its image read and processed. This invention also relates to a mechanism for opening and closing such a document holder with respect to the document table.
Optical image processing apparatus such as copiers are for reading and processing the image on an original document sheet and are provided with a document table adapted to have a document sheet placed thereon for a scanning operation. The document table is normally disposed at the top of the housing of the apparatus such as a copier of which it is a part and is usually made of a transparent glass material such that the document sheet placed thereon can be optically scanned by a light beam from the opposite side of the table. A document holder is attached to the housing of the apparatus below such that it can be opened from the document table and also can press a document sheet tightly against it when it is closed. In addition, the document holder serves to shield from external light the document sheet being scanned.
Some document holders are intended not only to hold an original in position but also to automatically transport a document sheet to a designated scanning position by means of a document feeder. The document feeder is generally disposed opposite to the document table and serves to automatically transport an original to be copied onto the table and discharge it after it has been processed. In general, many document sheets are stacked on top of a document supply table and they are transported onto the table one sheet at a time in response to a control command. After each sheet is moved to the scanning position and optically scanned, it is discharged into a discharge tray and the next sheet on the supply table is transported onto the table. Even those document holders incorporating such an automatic document feeder are provided with a hinge mechanism because some originals to be scanned cannot be fed automatically and must be manually brought to the scanning position by opening the holder.
Prior art document holders are attached to the apparatus below typically by means of hinges with one part affixed to its top surface along one edge of the document table, the other part being affixed to the document holder such that the document holder can be opened and closed around the pins of these hinges. The hinge mechanism is provided with biasing means such as coil springs for applying a biasing force in the direction of opening the document holder. In order to keep the document holder open at a desired angular position, however, cams and many other components are required and the prior art hinge mechanisms are generally very complicated. In addition, it is extremely troublesome to correctly adjust such springs such that their biasing force will exactly balance the weight of the document holder at the desired angle. If the adjustment is not correctly made and the gravitational torque on the document holder is greater than the biasing force of the springs, for example, the document holder cannot maintain an open position and starts closing. In order to maintain this balance, the positions of the sprigs must be carefully adjusted and the biasing strength of each spring must be correctly set. Document holders incorporating an automatic document feeder, in particular, are heavier and correspondingly stronger springs are required. Such document holders are therefore more complicated in structure and costlier to produce.
Another disadvantage of prior art hinge mechanisms for a document holder as described above has been that the biasing springs which are compressed when the document holder is closed tend to exert a diagonally upward force on the structure holding the document sheet and since this force is applied at the positions of the hinges, the document holder is subjected to a shearing stress. If the document holder becomes bent, not only does it fail to protect the document sheet from outside light but the document feeder therein becomes unable to correctly transport a document sheet to the scanning position.